1. Field of the Invention
The present invention relates generally to a threaded fastener.
2. Description of the Background Art
Generally, threaded fasteners having a portion for bit engagement in which six radial portions extend in radial directions are well known in the art.
Conventionally, a threaded fastener is composed of a head and a threaded shaft. In the head, an engaging portion, comprised of a groove or projection, is formed. The center portion of the groove or projection is circular to allow for insertion of a jointing equipment. Six radial portions are equally spaced around periphery of the center portion for holding a bit engagement with a jointing equipment to receive a torque derived from the equipment. A known threaded fastener of the above kind is shown in Japanese Utility Model Second Publication (allowed) No. 48-39959.
A plan view of the known threaded fastener is shown in FIG. 1. The threaded fastener is composed of a threaded shaft 1 and a head 2. The head 2 has a center portion A and six radial portions B extending in radial directions. The center and radial portions cooperate with each other to define a bit engaging portion 3. The bit engagement portion 3 is generated as follows. A root circle 5 is drawn which defines the center portion A. Six dummy circles 4 are equi-angularly spaced along the periphery of the circle 5. Another six dummy circles 6 having smaller diameter than the circles 4 are spaced therebetween, respectively. Two concentric circles 4' and 6' with the root circle 5 are drawn. The circle 4' interconnects the centers of the dummy circles 4, while the circle 6' interconnects the centers of the dummy circle 6. The profile of the portion 3 is formed along a line via a tangent point of the circle 4 with the root circle 5, parts of the circle 4 and parts of the circle 6. A plan view of another known threaded fastener is also shown in FIG. 2. In this type of threaded fastener, the profile of the portion 3 is determined along a line via tangent points of dummy circles 4 with the root circle 5, parts of the dummy circles 4, and parts of the dummy circle 7 drawn interconnecting the centers of the circles 4.
In the threaded fastener shown in FIG. 1, when a torque is applied to the bit engaging portion 3 by a jointing equipment, the torque converts a force W which works on the surface of each of the radial portions in a perpendicular direction. The force W is divided into two components forces F in a direction of a tangent line of circumference of the head and R in a radial direction. As a result, the force F is applied on the threaded fastener as a driving force. This driving force F is indicated by the following equation; EQU F=W cos .alpha.
wherein .alpha. is a driving angle defined by a tangent line l.sub.1 to the radial portion B at a point of force applied and a radius drawn l.sub.2 lined through this point. Therefore, corresponding to the reduction of the driving angle .alpha., a torque from the jointing equipment is converted to a rotating driving force F without loss. However, in this type of threaded fastener, the driving angle .alpha. becomes about 25.degree. or so, conversion loss of torque becomes large.
In the threaded fastener shown in FIG. 2, when a point of force applied is located most remote from the portion, that is where the circumference of the circle 7 meets the circumference of the circle 4, the driving angle .alpha. becomes zero, then the conversion loss of torque also becomes zero. However, practically, because the engagement member of the jointing equipment is relatively smaller than the bit engaging portion, the point of force applied is slightly removed inward of a theoretical tangent point at which angle .alpha. becomes zero. Therefore, the conversion efficiency of torque fluctuates slightly corresponding to fluctuation of the point of force applied. As a result, stable and high conversion efficiency can not be obtained according to these types of threaded fasteners. Additionally, because corners of the radial portion B are shaped having angles of 90.degree., corners portions tend to be fragile to stress concentration when the bit engaging portion is press-molded.